Hollow light permeable plastic plates surrounded by a frame can be used in horticulture for green houses and so-called light frames or starting frames as a substitute for window glass which has hitherto been used for such purposes.
It is known, for example, to provide rectangular light permeable plate elements as highly transparent hollow structures of highly transparent polycarbonate within frames of U-section aluminum profiles along the longitudinal and transverse edges of the transparent member and four plastic corner pieces interconnecting the frame limbs in pairs. The frame is thus constituted from four such aluminum profiles or shapes and four plastic elements and the hollow polycarbonate plate within the frame is held in the frame or the frame elements are held on the plate by mechanical clamping.
The polycarbonate plate itself may be fabricated by extrusion and can consist of two mutually parallel spaced apart flat outer layers on thin foils, and a plurality of mutually parallel transversely spaced ribs extending in the extrusion direction and interconnecting the outer layers and spacing them apart. The extruded high-transparency hollow plate of high-transparency polycarbonate, also referred to polycarbonate structural sheets, thus include numerous ribs which extend in the longitudinal direction, are spaced apart transversely to the extrusion direction and define between them the chambers, channels or hollow spaces, also referred to as cells, of the polycarbonate structural sheet. These cells are open at the ends of the polycarbonate structural sheet, which runs transversely to the extrusion direction.
The outer layers and ribs form the cell walls or the walls of the hollow spaces and because of the cellular structure give rise to a high intrinsic stiffness. However, there are problems with attaching the aluminum shapes forming the frame to such structures because of a certain flexibility at the edges of the outer sheets.
High transparency polycarbonate hollow plates are, by comparison with glass plates of equal thickness and size, significantly lighter and practically unbreakable, while having, because of their cellular structure, a high thermal insulation capability and reduced sound transmission.
The flexible cover layer and ribs may have a wall thickness in the range of 0.001 to 0.003 mm and frequently must be connected to aluminum profiles or shapes of U-section and wall thicknesses of 0.5 to 3 mm in forming the frame. The difference in the wall thicknesses and the different mechanical properties of the polycarbonate plate on the one hand and the aluminum frame limb on the other has meant that considerable effort must be made to lock the aluminum frame limbs to the light permeable plate so that it will not loosen in use and as a result of expansion and contraction effects with rising and falling temperature and exposure to solar radiation.
In practice, that has been done by bending fingers or projections from the aluminum frame limbs and so clamping the frame limbs on the light permeable polycarbonate plates that these members can penetrate into the cover layers of the polycarbonate. The members form hooks which can serve to anchor the frame members in place.
It is also known, in providing a mechanical connection between the polycarbonate structural sheet and an aluminum frame limb to press the free ends of the shanks of the metal profile to a smaller distance than the minimum plastic thickness of the polycarbonate plate. The edges of the hollow polycarbonate plate are then compressed between the shanks, causing the elastic ribs to buckle and provide an effective interconnection.
The frame can be assembled from aluminum members and plastic corners which can have pins or studs engaging in the frame limbs and also receiving the edge regions of the hollow polycarbonate plates so that a clamping action between the aluminum limbs and the plate is enhanced by the plastic corner pieces which can project through ribs of the polycarbonate plate for greater security of the mechanical connection.
In conventional plates with such frames, the aluminum U-section limbs enclose the edges of the hollow plates and thus close the open ends thereof, however, without effective sealing of them.
In practice, it is found that condensate can form within the compartments of the place or rain can penetrate into the compartments. Moisture can accumulate and in the presence of air, microscopic spores can grow which can reduce the transparency of the plate by forming dark colored deposits. Such algae and other biological growths cannot be removed without damaging the plate and without removing the frame members. In practice, it is found that once the frame member is removed, its replacement is not possible in a convenient manner.